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Descartes’s Substance Dualism vs Spinoza’s One Reality

Descartes’s Substance Dualism and Spinoza’s One Reality: Embodied Mind and Graded Consciousness, compared field by field from their entries in the Atlas.

At a glance

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Dualism Descartes’s Substance Dualism René Descartes
Neutral Monism Spinoza’s One Reality: Embodied Mind and Graded Consciousness Baruch Spinoza

The claim

In brief
Descartes’s Substance DualismRené Descartes holds that consciousness, which he calls thought (cogitatio), is the essence of an immaterial thinking substance (res cogitans) wholly distinct from extended, mechanical body (res extensa). Every conscious state, from reasoning and willing to pain, color, and emotion, is a mode of the thinking thing and is immediately present to it. Sensations and passions, though, arise only in the union of mind and body, where brain processes stand in regular relations to experiences they do not resemble.
Spinoza’s One RealityBaruch Spinoza holds that there is only one substance, God or Nature, known to us under two attributes, thought and extension. A human mind is the idea of an actually existing human body: one and the same thing as the body, conceived under thought. Because every body has a corresponding idea, mind runs through all of nature, and consciousness varies by degree with the complexity and power of the body.
What consciousness is
Descartes’s Substance DualismConsciousness is thought, defined as everything that happens within us insofar as we are immediately conscious of it, and it is the principal attribute of the mind.
Spinoza’s One RealityThe mind is the idea of the body, the same individual conceived under the attribute of thought rather than extension. Spinoza does not single out phenomenal consciousness as a category; his interpreters read consciousness as higher-order awareness, cognitive access, power or complexity of thinking, or affective awareness.

Big questions

Can AI be conscious?Differ
Descartes’s Substance DualismNoSince thought belongs only to immaterial substance and matter is pure extension, a machine, however complex, would be mechanism without a mind.
Spinoza’s One RealityConditionalSince every body has a corresponding mind whose richness tracks the body's complexity and power, a sufficiently complex artificial body would have a correspondingly rich mind; whether that mind would be conscious in the human sense is contested.
Does consciousness survive death?
Descartes’s Substance DualismYesDescartes presented the real distinction of soul and body as what makes the soul’s survival intelligible, and the Meditations announce that aim in their subtitle.
Spinoza’s One RealityNo position
Could a mind be uploaded?
Descartes’s Substance DualismNoCopying the body’s mechanism would not copy the immaterial thinking substance.
Spinoza’s One RealityNo position
Does it give life meaning?Both agree
Descartes’s Substance DualismCosmic/UltimateDescartes’s system rests on a non-deceiving God who guarantees knowledge and created even the eternal truths.
Spinoza’s One RealityCosmic/UltimateMinds are modes of God or Nature, and freedom lies in understanding the causal order of Nature through more adequate ideas.

Nature of mind

Ontology
Descartes’s Substance DualismSubstance dualism (Cartesian)
Spinoza’s One RealitySubstance monism (dual-attribute)
Mind and body
Descartes’s Substance DualismMind and body are really distinct finite substances with exclusive essences, thought and extension, and could by divine power exist apart. They are joined in the human being, where bodily motions give rise to sensations and the mind moves the body, though Descartes never explained that interaction satisfactorily.
Spinoza’s One RealityMind and body neither interact nor merely correlate; they are one thing under two descriptions. ‘The order and connection of ideas is the same as the order and connection of things’ (Ethics 2p7), so thought is explained only through thought and extension only through extension.
Fundamental or emergent
Descartes’s Substance DualismPrimitive
Spinoza’s One RealityPrimitive
Qualia
Descartes’s Substance DualismSensory qualities are not properties of matter, which has only size, shape, motion, and arrangement. Experienced redness or pain is a mental mode arising in the embodied union and cannot be captured by extension.
Spinoza’s One RealitySpinoza has no explicit theory of phenomenal character. Interpreters reconstruct it from his accounts of sensation, imagination and affect, where an affect is both a change in the body's power of acting and the idea of that change.

How it works

Mechanism
Descartes’s Substance DualismMechanism on the bodily side and non-reduction on the mental side. Light mechanically affects eyes, nerves, and brain, and the mind then experiences color, distance, and shape in regular correspondence with those motions.
Spinoza’s One RealityPsychophysical identity under parallel attributes: whatever happens in the body as extension happens in the mind as an idea, so no causal bridge between them is needed.
Where it is
Descartes’s Substance DualismThe mind is unextended. Sensations and passions are felt as belonging to one’s own body, and Descartes located mind-brain communication in the pineal gland.
Spinoza’s One RealityMentality is everywhere in nature, since all individuals are ‘animate’ in some degree (2p13s). Whether this means every physical thing has phenomenal experience is contested, and Spinoza does not treat God or Nature as one human-like conscious subject.
What it does
Descartes’s Substance DualismPassions are caused and strengthened by movements of the animal spirits and dispose the soul toward volition and action. The will can in turn move the body.
Spinoza’s One RealityMind never causes bodily motion and body never causes thought. Affects make ideas matter to behavior by increasing, decreasing or fixing the mind's striving, and freedom comes from forming more adequate ideas of the causes acting on us.

Evidence and criticism

Testability
Descartes’s Substance DualismConceptually testable
Spinoza’s One RealitySpeculative
Tests so far
Descartes’s Substance DualismNot stated
Spinoza’s One RealityNot stated
Criticisms
Descartes’s Substance DualismPrincess Elisabeth of Bohemia asked how an immaterial soul could move a material body, and the interaction problem remains unresolved. Arnauld and later critics charged circularity, since the proofs of God that guarantee clear and distinct perception themselves rely on clear and distinct perception. The standard reading denies consciousness to animals, which most now find implausible.
Spinoza’s One RealityThe identity formula says mind and body are one thing but not what makes the thought side felt. The ideas-of-ideas reading makes every idea conscious, which Bennett called absurdly excessive, and LeBuffe notes that a mind's overall power and the power of its ideas can diverge, giving odd verdicts on degrees.

Sources

Key sources
Descartes’s Substance Dualism
  • Meditations on First Philosophy, René Descartes (trans. J. Cottingham, R. Stoothoff, and D. Murdoch) (1984)
  • Principles of Philosophy, René Descartes (trans. J. Cottingham, R. Stoothoff, and D. Murdoch) (1985)
  • The Passions of the Soul, René Descartes (trans. J. Cottingham, R. Stoothoff, and D. Murdoch) (1985)
Spinoza’s One Reality
  • Ethics, Baruch Spinoza (E. Curley, Ed. and Trans., The Collected Works of Spinoza) (1677)
  • Spinoza’s Metaphysics: An Essay in Interpretation, E. Curley (1969)
  • A Study of Spinoza's Ethics, J. Bennett (1984)

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